Molecular mechanism of action of the synthetic retinoid N-(4-hydroxyphenyl)retinamide

Retinoids have long been investigated as chemopreventive and chemotherapeutic drugs for a variety of cancers. However, the use of the naturally occurring retinoids has been restricted due to unwanted side effects such as mucocutaneous irritation, night blindness, hyperlipidemia, headache, bone loss, and teratogenicity. As a result, several synthetic retinoids have been synthesized with the goal of improving potency and decreasing toxicity. This thesis study was conducted to investigate the molecular events mediating the biological actions of the synthetic atRA analog N-(4-hydroxyphenyl)retinamide (4-HPR). 4-HPR has been shown to prevent new tumor growth for a variety of cancer types in animals and in human clinical trials for the prevention of secondary breast cancer. Moreover, this synthetic retinamide has been reported to reduce tumor volume and number in rats with DMBA-induced mammary tumors. Numerous reports using a variety of cultured cancer cell types showed that 4-HPR induces cell death via apoptosis. Hydrolysis of 4-HPR back to the parent atRA has been proposed to account for its antiproliferative action but has not been proven. Comparison of 4-HPR with an unhydrolyzable analog, 4-HBR, showed that limited hydrolysis of retinamide does occur in vivo but cannot account for the anticancer action of 4-HPR. Also, this thesis study addressed the controversy regarding the role for retinoid receptor signaling in mediating growth inhibition by 4-HPR. Both 4-HPR and its carbon-linked counterpart were shown to induce RAR-mediated gene expression in several cancer cell lines (human breast, neuroblastoma, and promyelocytic leukemia) at concentrations that are also effective in promoting apoptosis. However, the selective blocking of RAR signaling using a pan-antagonist for all RAR subtypes was unable to affect apoptosis induced by 4-HPR or 4-HBR. Consequently, 4-HPR and 4-HBR induce cell death by an RAR-independent mechanism in cultured cancer cells. In addition, preliminary evidence is provided for mammary tumor regression via apoptosis in 4-HPR-treated rats.